Return Loss

What is Fiber Return Loss？

Return loss is the ratio of signal power injected from a source compared to the amount that is returned or reflected back toward the source. It is a critical performance parameter in both copper twisted pair and fiber optic cabling systems, because it can interfere with the transmitted signal and can contribute to an increase in the measured insertion loss (the amount of power that a signal loses as it travels along a cable link). If more power is reflected back to the source, less power is available at the far end of the cable. In some fiber optic systems, return loss can even damage the transceiver laser source.

The Formula for Calculating Return Loss

Measured in decibels (dB), return loss is calculated by comparing the input (or incident) power to the reflected power using the following formula:

Return Loss = 10*log (incident power/reflected power) in +dB

The result is always a positive number and a higher value is better. Consider that if none of the power from the source signal was reflected back, there would be an infinite return loss. A higher return loss generally correlates with less distortion in the transmitted signal.

Causes for Return Loss

Return loss in an optical fiber system is primarily caused by Fresnel reflections at connection points (i.e., connectors and splices). Dirty connector end faces are by far the most common cause, degrading return loss by 20 dB or more. Return loss can also be caused by poorly polished end faces, poorly mated connectors (i.e., air gaps and core misalignments), cracks in the fiber, open fiber ends, and impurities introduced into the fiber core during the manufacturing process. Micro and macro bends in the fiber that can occur as a result of installation stresses, such as exceeding bend radius or pulling tension requirements, can also affect return loss.

The angle of a connector end face can also have an impact on return loss. A UPC (ultra physical contact) connector end face is slightly rounded, while an APC (angled physical contact) end face is slanted at 8 degrees.

Return Loss Testing

Testing Tools

While an Optical Loss Test Set (OLTS) provides low uncertainty link and channel attenuation testing, field testing for return loss in a fiber optic system requires an OTDR that can measure the amount of light reflected back to the source. This is required for projects that specify extended (sometimes called Tier 2) testing in addition to attenuation testing.

OTDRs transmit high-power light pulses into a fiber, and when these pulses of light meet reflective events (i.e., connections, breaks, cracks, splices, sharp bends, or the end of the fiber), they are reflected back, traced, and characterized by the instrument. Return loss for a link is measured by calculating the total of all light reflected from all events and the total backscatter of the link. An OTDR also provides reflectance values and the location for each individual event, which is ideal for applications like short-reach single-mode, where you need to know the specific reflectance of connections, and for troubleshooting.

It is important to note that the use of an OTDR is considered an alternative test method. It does not replace the OLTS, because the total attenuation measurement achieved with an OTDR does not necessarily depict the total loss that will occur on a link once it is live.

Testing Procedures

Testing return loss with an OTDR requires the use of launch and tail cords, which allow for measuring the reflectance of the first and last connectors so they can be included in the overall return loss measurement. The lengths of the launch fiber and tail cord also need to be removed from the measurement via compensation. These OTDRs are easy to set up by simply selecting the fiber type and test limits, and then setting the launch compensation.

When using an OTDR, testing is done bidirectionally since the reflectance of specific connectors and splices depends on the test direction. Even if two connected fibers are of the same type, the fibers may have slight variations and different backscatter coefficients that can cause more light to be reflected after a connection than before a connection.

An OTDR plots reflected and backscattered light in a trace that graphically displays the characterization of a fiber link. Experienced OTDR users can typically recognize reflective events for launch cords, connectors, mechanical splices, fusion splices, mismatched fibers, and the end of the link. However, not everyone is a trace analysis expert. An OTDR features advanced logic that automatically interprets the trace and provides an ”EventMap” that indicates the location and reflectance of connectors, splices, and anomalies.